quick, acknowledging head nod done from head facing down to head facing up, all the while with you eyes facing the person you are nodding to.
done in a manner as to acknowledge a persons presence but at the same time in leu of a handshake or actual greeting. sometimes can be seen as a polite alternative to an all out snub (ie: ignoring the person who knows you saw them).

"Seen your homeboy tray the other day"
"did he say wassup?"
"naw we still kinda beefin so we just gave each other the nigga nod and kept it pushin."